Transmitter monitor



June 6, 1950 B. TREvoR ETAL TRANSMITTER MoNIToR Filed oct. s1; '1947 2 Sheets-Sheet l www .INN

v www G. MACLEAN A. ORNEY il INVENTOR {BERTRAM TREVOL BY Amun vvvvvv 2 Sheets-Sheet 2 QSKQ Q Y mi km. o L ,E STM Q .mm Es w mmm m 5e, E M o T s Ima. A T n R V 1 m m ,s m/ v Il Na n v 5S m Nuw v Tv B. TREVOR ETAL TRANSMITTER MONITOR June 6, 1950 Filed oct. 31, 1947 AAA Patented June 6, 1950 TRANSMITTER MONITOR Bertram Trevor and Kenneth G. MacLean, Riverhead, N. Y., assignors to Radio Corporation of America, a corporation of Delaware i AApplication October 31, 1947, Serial No. A'783,278

. Thi'srapplication discloses atransmitter monitor which is primarily for monitoring frequency shift telegraph transmitters by the operator at -the transmitter control point. Y

.. Heretofore, telegraphy signalling hasbeen carried on primarily by keying a carrieron and oif .in accordance with the signal. Monitoring such systems ,is .easy since all that is required is a simplerectifier excited by the transmitter output with the rectifier output lteredand applied to thevertical plates of an oscilloscope. Frequency shift telegraphy is coming into general use and these known monitors are entirely inadequate for monitoring the same. The primary purpose of our invention is to provide a monitor for analyz-V ing all `characteristics of frequency shift tele- `graphy waves.

vAt the transmitter it is essential to know the extent of frequency shift of the carrier bythe signal in order that thesaid shift can be regulated `to meet existing requirements with respect `to transmitter design vchannel assignments, andreceiver requirements. v

A primary purpose of our invention is to provide means giving an indication in cycles of the' peak to peak frequency shifts, which means operates continuously while the-transmitter is in use under traffic conditions.

It is also essential to provide a continuous visual indication of the wave shape, keying weightsand telegraph bias, if any, of the signal as represented by the modulation on the transmitted carrier. This indication is produced on the target of an oscilloscope and shows defects of the keyingwave such as transients.,

A further purpose of our invention is to provide driving potentials, which represent the modulation onthe transmitter output, -to control a tone keyer whose output may be sent over a tone line to a remote point or to a teletypeprinter and other circuits for monitoring purposes. This tone keyer is to be operated even in case our monitor receiver is detuned slightly.

The monitor of our invention is excited by in- ``termediate frequency output from a receiver,

1 which may be conventional, that must be properly tuned and our monitor also includes a tuning meter which shows the state of the receiver tuning.

A further object is to provide meter indications of the percent mark or keying bias of the transmitted carrier keyed by potentials lcontinuously reversing at a constant rate.

In order to adjust the apparatus used to attain the prior objects, we provide in the monitor a source of carrier current shifted by an accurately known amount which can befed tor-the apparatus in place of the received signal sentout by a transmitter. This wave insures accurate checking and. calibration, of the frequency shift indicator,

9 claims. (C1. 17a- 69) Inv order to properly adjust the signal keying level and other critical levels of the monitoring apparatus, we provide a keyer level meter which provides a continuous indication of the signal level. l

. ,A further object is to provide means for monitoring phase and/or frequency modulation of any kind on a transmitted wave.

In describing our invention, reference will be made Vto the attached drawings wherein the single figure (on two sheets) illustrates by circuit element and circuit element connections a monitor arranged in accordance with our invention.

`The signal fed to our monitor is derived from anyA communication type receiver having appropriate band width, sensitivity and the proper intermediate frequency. The signal to be monitored is tuned in on such a receiver (not shown) and the intermediate frequency output of the receiver is fed into jack J I which is the input jack of the monitor of our invention.

The intermediate frequency output in the example given Vfor descriptive purposes is at 455 kc. per second.

, The monitor is applicable primarily to frequency shift telegraphy and such a signal may be mpressed at J I. However, as described hereinafter, provision is made for monitoring frequency or phase modulated program transmitters.

Capacitor CI taken with capacitor C2 and wit inductor LI constitutes a broad band impedance matching input -circuit to the grid I0 of the intermediate frequency amplifier stage tube VI. The

anode I2 of tube VI is coupled by transformer T to the control grid I4 of tube V2 which is operated as a voltage and/or current amplitude limi iter. The transformer T has a primary inductor L2 with tuning capacitors C'I and C8 and a secondary inductor L3 with tuning capacitors Cl in the example described to have a flat lband width and CII. The transformer is coupled and tuned The resistor R5 connecting the grid I4 to gro-und taken in Vconjunction with the coupling capacitor CIZ forms a conventional grid biased limiting circuit. To maintain a calibration of the discriminator including tube V3 described 1 hereinafter, theintermediate frequency level at the input thereof should be held above the limiting value. The limiter tube V2 with its adjustable screen voltage supply source potentiometer R8 f and with the gain control in the receiver supply- ",irlg intermediate frequency energy to jack J I 1 screen grid I6 of tube V2 then is connected toa serve to hold the D.-C. level, as read on meter MI,

at the value used at the time of calibration. The

n movable tap on potentiometer R8` which is in a -voltage divider circuit across the high potential source.

The anode 20 of tube V2 is connected in 4an output circuit including in series inductors L4 and L5 which comprise respectively the primary Windingsof I,transformers T2 and' fT3. Theisecondary windings "of these transformers 'comprise inductors L and Li respectively shunted by tuning capacitors including adjustable capacitors C|8 and C2i respectively. The secondarymwndings of transformers T2 and T3 are each in a rectifier circuit including diodes-in amonimoni envelope in tube V3. Separate'diode` tubes may-be used. The discriminator tuned crcuitsiL'l and Lil- L6 may be mutually-,c o,upledl tojimproive the linearity of the discriminator. LE-Ll is tuned, in the exampleigiven, to 450 kc'ilidLB is tuned to 460 kc, providing a slope circuit havein'gpeaks separated' byf l0kc.

"U The-ioadfof the rectierfconnectedrwithinduc- #tor LE-isvresistor Rl i; lwhilefrthe load forthe `4reoitirier in-series WitirinductorLl comprises-resistor Rl2. The resistors are connected in opposed polarity with the connectionsA-suchfthat'-Ywhencurl`rente'flows Y through :itheresistors;- thea. potential drops therein are posi-tiveiatfthejunction-points betweeni-the resistors` andy` negative-at fthe-L ends *of the -resistorsconnected- -to-f the fanodes Aof #the diodes. The Aresistors fareA- shuntedl forf-voltages fiof-intermediatefrequency;and' -higherbyI capacitors 'f C24 and C25 -respectively. The "'-radio frequency input level meter MI is common tofthe ff-rectii'ierlcircuits; being connected between adjay cent terminals o1`- resistors R i ieandf4 Rl 2- and the cathodes of thediodes. *The vf'z'alibrated interme- "dia-te frequency levelis monitored-#byv the meter IMI-#which lreads theetotal''discrminatorecurrent. AnI-"output orf' Vload circuit,f comprising Vresistors -RM'rR-i 5, Ril E' and Ri if, -iscsonnected inseries vvacrossV the rectifier load resistors -RI l- -and R12. LAA switch VS l is-arranged; yto connect afl-se1eeted point onfthedetectoroutputfcircuit-to a Vfre- -quency shift voltmeter-"tube' V4 0 described here- "in'aften The originalmodulation intelligenceof 'i Vavfrequencyfshiftedeor frequency modulated-signal-appears across the resistors-Ri i to Ri 'iinclusive and as aA cor-isequeI-ic'epaerossf-R-"idinl shunt ii'thereto.

point on potentiometer`- sresistor- Rl 8l -isf conneet-'ed -by-resistor= RI 9-tok 5vthecontrol grid-e30 of r an electrondischarge dev-ice -VAWhen Vtheswitch A'52h-isdn' the po'sMiou-shown.y The tube 'VAis a cathode driverwithcathode load1resistorR-2i and Aisolates ther d'iscriminator-f and detector f-ronrthe "various outputcircnits now'beingdescribed. A The 'cathode driver' :stage ir'icluding"-tube`-"Vfl`A suppl-ies y output'for Lour purposes. Its` cathode- MrA is icon- Anectedto `a,-'tuning voltmet-erY tubefiV, by lead' 4l2 bto'bthercontrol gridsA of a; doubley tube'system *V8 and by resistors R2i and R22 to an output-lead fl43labeled tone keyer. Y

The outputv ofi the cathodedriver -tube VA' f ed ";'to"tube V5 isused to 1insure proper tuning` or-the freceiver to the centers-of the discriminator-icir- --cuitrcharacteristic il This' is accompl-ishedby suplpl-yingV current representing-the discriminator and rvdetector outputto a tuning -meterfiMZunlfthe *cathode-circuit of va@tubelVllB-v'w-hich maybe *in a common envelope- Wit'n thetube-'VAbr may-be inv aseparate envelope. rEhe cathode 40 z'of f--tube Y AiVllA is connected directly-to=the1anodeinone of the diodes -of tube Va-and-also directly to Lthe -lcathodev-'off'the other-:off the `diodes tube V5. feline-cathode and Y anode respectivelyiof,V the :said

-lfdiodes fof this voltageldoublerz rectifier are :con-

i.nectedaito arectifier@- load comprisingy resistors R28 to R3| shunted by ltering capacitors'iGSl Hand C32. lfWith'frequency shiftl keying having mark and space frequencies equally spaced about the discriminator center, the voltage of cathode of tithe tubelwiaeisrfkeyed equal varnounts'eabove'and below afpotenti'al' correspond- 5Y ing to the discriminator center frequency. The "'ftwo diodes of V5 each maintain across their regvspecti'vexfload resistors R28, R29 and capacitors G3i, C32 in opposite polarity, the peak values of -`-ithe voitage=excursions of cathode 40. Therefore,

jloiwi-th-'keyingdcntered in the discriminator, the

"junctiontoi equaresistors R39, R3i gives a po- -tentiaigiat-egridiziof tube VHB, equal to the discriminator center voltage o1" cathode 46. With *keying not centered inithe discriminator cathode has unequal voltage excursions about the center:pote'i'itial:l Whchiproducesat: grid:` .tainew f D-Cnfpotentiall'aboveior zbelow; thea-center: valve, depending; uporriwhetheiiuthez.keying i isfdisplacied above or below .thediscriminatorzrcenter YJremyquency.' Thesjunction point-1`of+resstorsrR3tand R3 i'i-.is connected; inl-fthe switchsSB position I -shownntoi lthe-- -control .gridsi' fof.' thertuning meter f1tubeVB,-yvhich:has the meter lvaineitsr'cathode return circuiti..in-'aserieswith resistors-R32 Note *thatV switchesv SiAc-a'nd 52B` are: ganged.'..-.Theid ode-'V51 Withf itsfresistorzload an'dbypass; capaci- =tors1isafpeaketofpeakafiloltmeter` :Withflslowf time constants; -sothatlithe frequency-shift.Y telegraphy Vilsignalso variable; weight oriperce'nrt mark: set @up a;'steadybiask1 onthe' gridi;` Whi'chcausesrsteady current-to iiow-2ilffftliermeteM2y when-.':frequency shift or similar telegraph signals'of variable Weight-"ars ed'to'rth'e discriminator.

Y-ITiie anodeief'of thedoublezdiodejin clampitube Vis- -conriectdftothe Atap ionllresistors. Rgand i R3 iilwhilei'the-catrrode oflthisl=tubelv-isiconnected @tof a-i potential' dividen-'including :resistorsJ` R34 and R35. The biases areisuchthatifi thefdirect Y "'curr'entipotentialconethe fanodefs lli-"-andtnat.` the 40-ipoi1`it betweenfvthefresistorslR3l.:and R'Ssbecomes exc'essi'velyipo'sy lyeiithe vIdibdervtube Willi-conduct i tot-dischargeandlflowerthis potential. if Inffths manner, the tube ViBiisiprotectedI-from:excessive grid'potentials'a'ridfthelfadjustnient isfsuch, in the 'feerampl igivengffthat the; tube 6 -conductsfwhen sthe-potenti'alionf-the grid@ wexceeds -l-ivolts.

-`fAnothereoutpuU-fromi the cathode-Lof the cathode fdriver tubeldk isithrough'sresistor-LRZ l candi-'capacito- G3 .tm a' ert'onei--keyer. externally V located, f"ifl^fdesir-ed.f i rEhe tonefkeyerfetubel" grid -(not -`sliownr-providesaficlamp 1to1; limit the .posi- -tive -voltage fedez-toitheikialyer .toi aboutzera 4volts while 'the'flower of 'i'fhelclampvy tube 6l 4upre- -fyents'the tone ke'yeirlfrorn. receivingrmorethan :rn'inusE 'ivoltsAso-that'finspitefo moderate de- --tuning=f =ofithe signa "idue-i-torthewreceiverifdrift, Va-f'constant level is fed'J-to thef-tone keyer Bhe protection is'suppliedibythe tone `keyer grid `with respecte-to positive voltages Lbe'cause-'fif'."^-.the grid otendsto (swingipostiveg gridrrectficaton takes -placerto make the:gridipotentialinegative -lnegativeivlevelf-is xce'ssivalV the catlrode'i.i "clamp:v tube V6 bcomeslnegative relative :t the anode 63 and discharge takes place to limtthe 65" h'egatVB-DKS I'1L-By niovinglithefswitches ISZAland-ffSBeto the enumber ..twoe` anditinteespotionsriandgdjnsting resistors R33 andsf'md-Itheibiasf ofi tubesElMB U."an'dfVfLi :can bei .made fsuchf'fa's: to;x bningftuning i=2meter-f-M2 lpointerrtor centerliscalerl withV noijsignal inputand /oreWithsignal`-'.inputf:properly:r tuned L tiltheeenter `frequencyhof fthe"` discriminator; In order Ito;provideifsymnietrical'f-DCkeying o; the ktonef-fkeyenovefleadillii itivi .fdesired thatathe centeraor? base potential ofxca'thod'f bbefmaintained at -2 voltsin the example given. Un-

der this condition tuning meter M2 is adjusted by means of R33 to read center scale after setting switch S2B to position two, thereby connecting grid 50 of tube V4B to a xed potential of -2 volts from the potential divider R23 to R21. Position 3 of switches 2A andV 2B permits adjustment, by means of resistor R24, of the potential of grid 38 of tube V4A, which in turn causes the potential of cathode 4i] to vary in a similar manner. Resistor R24 is adjusted in this case to give a potential at cathode 4t of -2 Volts, which `is indicated by a center reading of meter M2. Now with the switches returned to the number one position, the receiver may be tuned, if necessary, to supply at jack JI an average intermediate l frequency which is centered on the discriminator characteristic, this being indicated, by zero or center scale reading on the meter M2. a The output of tube VilA appearing across resistor R25 is alsosupplied by lead 42 and capacitor C42 to the grid 19 of the right hand section of a double tube V8 which has two output load `impeders, R43 adjacent the cathode and R44 adjacent the anode, both in the direct current path of tubes V8. The right hand section of this tube V8 thus operates as a phase inverter so that the output at the anode may be considered positive and the output at the cathode may be considered negative and these plus and minus signals may be supplied by reversing switch S4 and lead 12 through a condenser C43 to a monitoring oscilloscope, not shown. i The output from the cathode driver V4A also is coupled through lead 42 to the control grid 14 of the left hand section of tube V8, which is a cathl ode'driver tube supplying output from its cathode resistor R4I through capacitor C4 I' to the control grid 15 of the left hand tube section of tube V9 used with percent mark meter M3 and also to a rectifier 18 supplying output to the control grid 80 of the other section of the tube V9 which has a i keyer level meter M4 in its cathode return.

The left hand section of tube V9 with meter M3 is used to facilitate adjustment of the keying weight or percent mark atv the transmitter. To do this, the transmitter must be keyed by constant frequency reversals (not necessarily 50% mark) and the modulation circuits are adjusted for the desired percentage mark which may be changed through a range of 20 to 80% as indicated on the meter M3 whenthe transmitter signal is fed to jack J l. Then the current flowing in the meter M3 will be of a magnitude depending upon the percent mark. Frequency shift 80%.- `In establishing this operating condition,

l envelope.

= telegraphy signals normally use a shift to higher frequencies to represent mark. The polarity of the discriminator is chosen such that this produces a negative going voltage formark at the grid 16 of the left hand section of tube V9.y The cathode 11 of this section of tube V9 is grounded f through meter M3 so that the grid cannot be madematerially positive relative to the cathode. the presence of mark the potential at the left .hand cathode of tube V8 swings in a negative direction. Therefore, the grid to cathode impedance of the left hand section of tube V9 acting as a diode will conduct only on positive going or space signal intervals andthe average D.C. voltage developed onthis grid will vary with the relative length of the mark and space intervals. The meter M3 readings follow the average grid voltage. The resistor R41 is chosen so that .a plate current change of 7.6 milliampere. occurs when the percent mark is vvaried from `20% to the bias on the grid 16 is also selected as desired by grid bias resistor R45. A compensating current for meter M3 is supplied by resistor R46 and R50 connected to a negative point on the'lpotential divider described hereinbefore comprising resistors R23, R24, R25, etc. Resistor R46 is variable and is adjusted to give .5 milliampere in the meter M3 with 50% mark. This mark meter Vreads the percent mark directly from about 20% to mark.

The left hand grid-cathode section ofv tube V9 with the crystal rectier 18 and capacitors C4I and C44comprise a voltage doubler rectier or peak-to-peak rectifier for the keying wave The keying wave appears across the cathode load R4! of the left hand section of tube V8 and is fed from this load through capacitor C4I to the grid 16 and the cathode of the rectifier 18, the anode of which is connected to the control grid 80. The negative going potentials are rectified in the rectifier 18. The positive going potentials are rectified in the grid circuit of the left hand section of tube V9. A potential representing the rectified peak-to-peak keyed wave envelope appears at the anode of rectifier 18 and across load resistor R49 shunted by filtering capacitor C44 and is applied to the control grid 88 of the keyer level meter tube. The average strength or peak-to-peak level of the keyed signal appearing across R4I is registered in the meter M4 in the cathode return circuit of the right hand section of tube V9. This meter is a means for monitoring the critical levels involved in performing the variousoperations outlined above. This level is kept at the correct value by adjustment of potentiometer resistor RI 8 at the discriminator detector output.

The left hand section of tube VII) is a cathode driver stage having its control grid 90 coupled by the switch SI in the position shown to a point on the voltage divider across the output of the discriminator detector and this tube has )its cathode load resistor R5I coupled by the switch S5 in the position shown to a coupling capacitor `C45 in turn coupled to rectiers 96-96' supplying the grid |99 of the right hand stage of tube VIG which has in its anode circuit a meter M5 which indicates the total shift of a frequency shift telegraph signal. The cathode driver section load resistor R5I is connected to a negative point on the potential source while the anode of the left hand section of tube VIIJ is connected to a positive point on this source. The frequency shift signal supplied to the grid 90 from the discriminator detector output appears across the load resistor R5I and is fed by coupling capacitor C45 to the anode and cathode respectively of the two crystal diodes 96 and 96. The diodes 96 and 96 with capacitors C45 and C46 form a voltage doubler rectifier. The positive going potentials are rectified by the rectifier 96. `The negative going potentials are rectified by the rectifier 96. The rectified signals appear on the control grid IUD of the right hand section of tube VI 0 to vary the bias thereon and thereby vary the plate current in this tube and the current flowing through the total shift indicator meter M5, in proportion to the peak positive to negative excursions of the signal envelope appearing in the discriminator detector output. The anode I02 of this section of tube VII) is connected by resistor R53 to a positive point on the direct conventional, a detailed description thereof other than that above will not be given.

No calibration system is incorporated for monitoring for phase or frequency ,modulation but such a calibration may `be made by feeding the output of the right hand section of tube V!! to a db pad and a volume indicator system. With a known degree of phase or frequency modulation input the volume indicator is read. Values of modulation relative to this level may then be read on the indicator. e

The power supply circuits and connections have in a large part been left undescribed since they are conventional and are not shown in the drawings and are understood to those versed in the art.

What is claimed is:

1. In frequency shifted wave analyzing means, a detector excited by current the amplitude of which is shifted in a manner corresponding to shifts inthe wave frequency, a potential divider coupled to the output of said detector, at least two connections to said divider at points where the ratio of the potentials appearing thereat, when the detector is operating, is known, a peakto-peak potential reading voltmeter, and a switch to connect said voltmeter to a selected one of said two connections.

2. In a monitor for analyzing wave energy the frequency of which is shifted between two values in accordance with signals, in combination, an amplifier for amplifying said wave energy, a frequency discriminator and detector coupled to said amplifier for demodulating said wave energy to produce voltage variations corresponding to the frequency shifts of said wave energy, a'peak-topeak voltmeter coupled to said detector for producing indications of the variations of the produced voltages and consequently the extent of frequency shifts of said wave energy, and apparatusfor Calibrating said voltmeter comprising an oscillation generator of a frequency substantially equal to the frequency of the wave energy, means for shifting the frequency of the generated oscillatory` energy through a known range at a recurring rate, and means for feeding the frequency shifted oscillatory energy tothe input of said amplier.

3. A monitor as recited in claim 2 including a current meter in a direct current circuit of said detector for checking the operation of the discriminator and detector.

4. In a monitor for analyzing wave energy the frequency of which is shifted between two values in accordance with signals, in combination, an amplifier for amplifying said wave energy, a frequency discriminator and detector coupled to said amplifier for demodulating said wave energy, and impedance coupled to said detector, in which impedance voltage variations corresponding to the frequency shifts of said wave energy are produced, a voltage divider comprised of at least two resistors in shunt to said impedance with said resistors so proportioned that the voltage drop across one resistor is a known factor of the voltage drop across the other resistor, a voltmeter for producing indications of the peak-topeak variations of the produced voltages and consequently the extent of frequency shifts of said wave energy, a switch for coupling said Voltmeter to one or the other of said resistors, and

apparatus for Calibrating said voltmeter comprising an oscillation generator of a frequency substantially equal to the frequency of the wave energy, means for shifting the frequency of the generated oscillatory energy through a knownv range at a recurring rate, and Vmeans for feeding the frequency shifted oscillatory energy to the input of said amplier.

5. A monitor as recited in claim 4 wherein said voltmeter indicates average current intensity and wherein a full wave rectier and voltage doubler is in the coupling between said one or the-other of said resistors and said voltmeter.

6. In a monitor for analyzing wave energy the frequency of which is keyed between two values in accordance with mark and space signals, in combination, a tunable amplier for amplifying said wave energy, a frequency discriminator and detector coupled to said amplifier for demodulating said wave energy, an impedance connected to said detector in which impedance voltages keyed in a manner corresponding to the frequency shifts of said wave energy are produced, a cathode driver stage coupled to said impedance, said cathode driver stage having an output impedance wherein voltage variations keyed in a manner corresponding'to variations in the frequency of said wave energy appear, a current intensity meter, a full wave rectier coupled to said second-named impedance, said full wave rectifier having opposed output impedances shunted by capacitors, the time constants of the last named impedances and capacitors being long with respect to the keying rate so that a bias is developed at said last-named impedances which is dependent on the tuning of said amplifier irrespective of the percent mark, and a connection between said meter and said last-named impedances.

7. A monitor as recited in claim 6 wherein said last named connection includes an electron discharge devicehaving input electrodes coupled to said last named impedances and having output electrodes in a circuit including said meter.

8. A monitor as recited in claim 7 including a positive voltage peak limiter in shunt to the input electrodes of said device.

9. In apparatus for producing indications of the peak deviations of frequency shifted energy, in combination, means for producing varying potentials the peak values of which change in a manner corresponding to the deviation peaks of said wave energy, an electron discharge device having input and output electrodes, a meter in circuit with the output electrodes of said device, a full-wave rectifier coupling said means to the input electrodes of said device, a source of oscillatory energy of known frequency, modulated through a known range at recurring intervals, and means for feeding said modulated oscillatory energy to said first-named means in order to calibrate said meter.

BERTRAM TREVOR. KENNETH G. MACLEAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

